High altitude platforms (aircraft and lighter than air structures situated from 10 to 35 km altitude)—HAPS, have been proposed to support a wide variety of applications. Areas of growing interest are for telecommunications, positioning, observation and other information services, and specifically the provision of high speed Internet, e-mail, telephony, televisual services, games, video on demand, mapping services and global positioning.
High altitude platforms possess several advantages over satellites as a result of operating much closer to the earth's surface, at typically around 20 km altitude. Geostationary satellites are situated at around 40,000 km altitude, and low earth orbit satellites are usually at around 600 km to 3000 km altitude. Satellites exist at lower altitudes but their lifetime is very limited with the consequent economic impact.
The relative nearness of high altitude platforms compared with satellites results in a much shorter time for signals to be transmitted from a source and for a reply to be received (the “latency” of the system). Moreover, HAPS are within the transmission range for standard mobile phones for signal power and signal latency. Any satellite is out of range for a normal terrestrial mobile phone network, operating without especially large antennas.
HAPS also avoid the rocket propelled launches needed for satellites, with their high acceleration and vibration, as well as high launch failure rates with their attendant impact on satellite cost.
Payloads on HAPS can be recovered easily and at modest cost compared with satellite payloads. Shorter development times and lower costs result from less demanding testing requirements.
U.S. Pat. No. 7,046,934 discloses a high altitude balloon for delivering information services in conjunction with a satellite.
The patents, US 20040118969 A1, WO 2005084156 A2, U.S. Pat. No. 5,518,205 A, US 2014/0252156 A1, disclose particular designs of high altitude aircraft.
However, there are numerous and significant technical challenges to providing reliable information services from HAPS. Reliability, coverage and data capacity per unit ground area are critical performance criteria for mobile phone, device communication systems, earth observation and positioning services.
Government regulators usually define the frequencies and bandwidth for use by systems transmitting electromagnetic radiation. The shorter the wavelength, the greater the data rates possible for a given fractional bandwidth, but the greater the attenuation through obstructions such as rain or walls, and more limited diffraction which can be used to provide good coverage. These constraints result in the choice of carrier frequencies of between 0.7 and 5 GHz in most parts of the world with typically a 10 to 200 MHz bandwidth.
There is a demand for high data rates per unit ground area, which is rapidly increasing from the current levels of the order 1-10 Mbps/square kilometer.
To provide high data rates per unit ground area, high altitude unmanned long endurance (HALE) aircraft, or free-flying or tethered aerostats, would need to carry large antenna(s) to distinguish between closely based transceivers on the ground. A larger diameter antenna leads to a smaller angular resolution of the system, hence the shorter the distance on the ground that the system can resolve. Ultimately the resolution is determined by the “Rayleigh criterion” well known to those skilled in the art. The greater the antenna resolution, the higher the potential data rates per unit ground area are.
Furthermore having a large diameter antenna tends to reduce sidelobe levels from the antenna and this can be an important consideration when operating near the borders of countries with different allocations of spectrum.
Phased array digital “beamforming” (DBF) antennas have been considered for HAPS in for example, R. Miura and M. Suzuki, “Preliminary Flight Test Program on Telecom and Broadcasting Using High Altitude Platform Stations,” Wireless Pers. Commun., An Int'l. J., Kluwer Academic Publishers, vol. 24, no. 2, January 2003, pp. 341-61. Other references include: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=620534, http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=933305, http://digital-library.theiet.org/content/journals/10.1049/ecej_20010304, http://digital-library.theiet.org/content/journals/10.1049/el_20001316 http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4275149&pageNumber %3D129861 HAPs have a limited capacity to lift payload and provide power. This normally limits the antenna size even if the aircraft platform can accommodate a larger diameter of phased array.
At lower altitude, user equipment antennas, if of suitably large diameter, can send and receive signals from different HAPS without significant interference operating at a common frequency. This is described in David Grace, John Thornton, Guanhua Chen, George P. White, Tim C. Tozer: Improving the system capacity of broadband services using multiple high-altitude platforms. IEEE Transactions on Wireless Communications 4(2): 700-709 (2005).
The abstract of this paper provides a summary:
“A method of significantly improving the capacity of high-altitude platform (HAP) communications networks operating in the millimeter-wave bands is presented. It is shown how constellations of HAPs can share a common frequency allocation by exploiting the directionality of the user antenna. The system capacity of such constellations is critically affected by the minimum angular separation of the HAPs and the sidelobe level of the user antenna. For typical antenna beamwidths of approximately 5 degrees an inter-HAP spacing of 4 km is sufficient to deliver optimum performance.
The aggregate bandwidth efficiency is evaluated, both theoretically using the Shannon equation, and using practical modulation and coding schemes, for multiple HAP configurations delivering either single or multiple cells. For the user antenna beamwidths used, it is shown that capacity increases are commensurate with the increase in the number of platforms, up to 10 HAPs. For increases beyond this the choice of constellation strategy becomes increasingly important.”
Transmission and reception of much higher data rates between individual user equipment can in principle be achieved by using multiple HAPS in a “constellation” where signals to and from the HAP antennas illuminate the same area but the user equipment antennas are able to distinguish between the different HAPS. The data rate can be at a multiple less than or equal to the number of HAPs in line of sight to that which could be achieved by a single HAP with a defined bandwidth, with the exact multiple determined by the signal to interference plus noise ratio at the receiver.
For such a system, there is benefit in using existing mobile, rather than the mm wavelengths referred to in the paper by Grace et. al.: frequencies (typically above 0.6 GHz to 4 GHz-50 cm to 7.5 cm wavelength—λ), due to their relatively low absorption and better penetration through walls and other objects. Higher frequencies up to 40 GHz (7.5 mm wavelength) can also be utilised if there is a clear line of sight. However at all frequencies referred to, the size of a conventional antenna needs to be substantial in order to spatially differentiate between neighbouring HAPs if they are close together as would be required for system capacity significant benefit.
As described in the paper referred to above by Grace et. al. (2005), beamwidth from the user equipment ground based antenna of only a few degrees is desirable if there are many (˜16 or more) HAPS in line of sight. An antenna to give this resolution needs to be of diameter (D) around 35× the wavelength used (Rayleigh criterion, angular resolution˜1.22λ/D). At the most commonly used mobile frequencies (˜1 to 2 GHz) this leads to a required antenna diameter of over 5-10 m in diameter, and even at 20 Ghz leads to an antenna diameter of around 0.5 m.
Making a steerable dish antenna of such a size requires a large volume and is expensive. Furthermore if multiple HAPs need to be tracked, either the detector/transmitter arrangement near the focal point of the antenna becomes complicated and practically difficult, or multiple dish antennas are needed adding to the expense.
If many HAPs are to be used, spatial resolution to achieve high data rate alternatives to a conventional user equipment dish antenna are required—allowing ideally a lower expense and lesser volumes.
In the paper (Grace et al—2005), the user equipment referred to is ground based. However the principles also apply to elevated user equipment beneath the HAPs.
In the following description user equipment can be ground based or at an altitude lower than the high altitude platforms.